Bose Corporation v. Beats Electronics LLC et al

Filing 1

COMPLAINT filed with Jury Demand - against Beats Electronics International Limited, Beats Electronics LLC - Magistrate Consent Notice to Pltf. ( Filing fee $ 400, receipt number 0311-1560024.) - filed by Bose Corporation. (Attachments: # 1 Exhibit 1, # 2 Exhibit 2, # 3 Exhibit 3, # 4 Exhibit 4, # 5 Exhibit 5, # 6 Exhibit 6, # 7 Exhibit 7, # 8 Exhibit 8, # 9 Exhibit 9, # 10 Exhibit 10, # 11 Civil Cover Sheet)(rwc)

Exhibit 5

UNITED STATES DEPARTMENT OF COMMERCE
United States Patent and Trademark Office
July 09, 2014
THIS IS TO CERTIFY THAT ANNEXED HERETO IS A TRUE COPY FROM
THE RECORDS OF THIS OFFICE OF:

U.S. PATENT: 8,054,992
ISSUE DATE: November 08, 2011

By Authority of the
Under Secretary of Commerce for Intellectual Property
and Director of the United States Patent and Trademark Office

M. TARVER
Certifying Officer

111111

1111111111111111111111111111111111111111111111111111111111111
US008054992B2

United States Patent

(10)

Sapiejewski

c12)

(45)

(54)

HIGH FREQUENCY COMPENSATING

(75)

Inventor:

(73)

Assignee: Bose Corporation, Framingham, MA
(US)

( *) Notice:

EP
GB

wo

Roman Sapiejewski, Boston, MA (US)

CN Office Action dated Dec. II, 2009 for CN Appl. No.
200710I04498.6.
Roman Sapiejewski, ''Active Noise Reduction Microphone Placing,"
Patent Application.
Yu, Shiang-Hwua eta!., Controller Design for Active Noise Cancellation Headphones Using Experimental Raw Data, IEEEIASME
Transactions on Mechatronics, vol. 6, No.4, Dec. 2001.
Partial EP Search Report dated Mar. 4, 2009 for related EP Application No. 07I06779.7-I240 I I850631.
Extended EP Search Report dated May 27, 2009 for EP Appl. No.
07106779.7-I240.
Extended EP Search Report dated Feb. I7, 2009 for EP Awl. No.
07106780.5-I224 I I850632.
EP Office Action dated Oct. 8, 2009 for EP 07I06780.5-I224 I
I850632.
EP Notice of Allowance dated Jun. 11, 20IO for EP 07I06780.5.

Appl. No.: 11/409,894

(22)

Filed:

(65)

Apr. 24, 2006
Prior Publication Data

US 2007/0253567 Al
(51)

'(52)
(58)

Nov. 1, 2007

Int. CI.
H04B 15/00
(2006.01)
H04R 3/00
(2006.01)
H04R 1140
(2006.01)
GJOK 11116
(2006.01)
U.S. Cl................. 381/93; 381/95; 381/96; 381197;
381/98; 381/71.6
Field of Classification Search ................. 381/71.6,
381/93, 95, 96, 318, 97, 98; 330/109,294
See application file for complete search history.

* cited by examiner
Primary Examiner- Devona Faulk
Assistant Examiner - George Monikang
(57)

U.S. PATENT DOCUMENTS
3,759,063
4,494,074
5,I82,774
5,38I,485
5,809,I56
5,949,897
200510259833
200710253567

A
9II973 Bendall
A
1/I985 Bose
A
III993 Bourk
A
1/I995 Elliott
A
9II998 Bartels et al.
A
9II999 Bartels et al.
AI* Il/2005 Scarpino eta!. ................ 381/98
AI
Il/2007 Sapiejewski

FOREIGN PATENT DOCUMENTS
0333411
0688I43
I39899I

EP
EP
EP

ABSTRACT

A method and apparatus for increasing phase margin in a
feedback circuit of an active noise reduction headphone. The
method includes providing an acoustic block comprising an
acoustic driver comprising a voice coil mechanically coupled
along an attachment line to an acoustic energy radiating diaphragm, the acoustic block further comprising a microphone
positioned along a line parallel to an intended direction of
vibration of the acoustic diaphragm and intersecting the
attachment line, the acoustic block characterized by a magnitude frequency response compensating the magnitude frequency response by a compensation pattern that has a positive
slope over at least one spectral range above 10kHz.

References Cited

(56)

IOI2005
I21I993
III989

I587342
2267411
8900746

OTHER PUBLICATIONS

Subject to any disclaimer, the term of this
patent is extended or adjusted under 35
U.S.C. 154(b) by 1440 days.

(21)

Patent No.:
US 8,054,992 B2
Date of Patent:
Nov. 8, 2011

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1

2

HIGH FREQUENCY COMPENSATING

a compensation pattern that has a positive slope over at least
one spectral range above 10 kHz.
In another aspect, an active noise reduction apparatus
includes an acoustic driver. The acoustic driver includes a
diaphragm and a voice coil, for applying mechanical force to
the diaphragm along a force application line; a microphone
with a microphone opening positioned within 2 mm of a line
parallel to an intended direction of motion of the diaphragm
and intersecting the force application line; and structure for
attenuating frequency response aberrations resulting from
resonances of components of the acoustic driver. The apparatus also includes an acoustic block characterized by a first
magnitude frequency response and a compensator characterized by a second magnitude frequency response to combine
the second magnitude frequency response with the first magnitude frequency response to provide a combined magnitude
frequency response. The second magnitude frequency
response is characterized by a pattern that has a positive slope
at a frequency interval in the spectral portion above 10kHz.
Other features, objects, and advantages will become apparent from the following detailed description, when read in
connection with the following drawing, in which:

BACKGROUND
This specification relates to feedback control in an active 5
noise reduction headphone. Reference is made to U.S. Pat.
No. 4,494,074, Bose, "Feedback Control."
SUMMARY
10

In one aspect of the invention a feedback circuit for an
active noise reduction headphone includes acoustic elements
characterized by a first magnitude frequency response; a
compensator characterized by a second magnitude frequency
response to combine the second magnitude frequency
response with the first magnitude frequency response to provide a combined magnitude frequency response, wherein the
second magnitude frequency response is characterized by a
pattern that has a positive slope at a frequency interval in the
spectral portion above 10kHz. The feedback circuit may have
a positive slope between 20kHz and 50 kHz. The pattern may
have a positive slope between 20 kHz and 100 kHz. The
compensator may include a digital filter. The compensator
may include an analog filter.
In another aspect, a method includes, in an active noise
reduction headphone characterized by a magnitude frequency
response, compensating the magnitude frequency response
by a pattern that has a positive slope between 20 KHz and 50
kHz. The compensating may include compensating the magnitude frequency response by a pattern that has a positive
slope between 20 kHz and 100 kHz.
In another aspect, a compensation pattern for an active
noise reduction headphone is characterized by a positive
slope in the frequency range between 20 KHz and 50 kHz.
The compensation pattern may be characterized by a positive
slope in the frequency range between 20KHz and 100kHz.
The compensation pattern may be characterized by a greater
than 2nd order positive slope between 20kHz and 100kHz.
In another aspect, a compensation pattern for an active
noise reduction headphone is characterized by a positive
slope above 10 kHz for a range of at least one octave. The
compensation may be characterized by a positive slope for a
range of at least two octaves. The compensation pattern may
be characterized by a positive slope for a range of at least three
octaves.
In another aspect, a method includes providing an active
noise reduction headphone characterized by a magnitude frequency response and compensating the magnitude frequency
response by a pattern that has a positive slope in at least a
portion of the spectral range above 10 kHz for at least one
octave. The compensating may include compensating the
magnitude frequency response by a pattern that has a positive
slope above 10kHz for at least two octaves. The compensating may include compensating the magnitude frequency
response by a pattern that has a positive slope above 10 kHz
for at least three octaves.
In another aspect of the invention, a method for increasing
phase margin in a feedback circuit of an active noise reduction
headphone includes providing an acoustic block that includes
an acoustic driver. The acoustic driver includes a voice coil
mechanically coupled along an attachment line to an acoustic
energy radiating diaphragm. The acoustic block further
includes a microphone positioned along a line parallel to an
intended direction of vibration of the acoustic diaphragm and
intersecting the attachment line. The acoustic block is characterized by a magnitude frequency response. The method
includes compensating the magnitude frequency response by

15

20

DESCRIPTION
25

30

35

40

45

50

55

60

65

FIG. lA is a view of noise reduction headphone;
FIG. lB is a block diagram of a logical arrangement of a
feedback loop for use in the headphone of FIG. lA;
PI G. 2A is a diagrammatic top view of an arrangement that
reduces time delay between the radiation of acoustic energy
by an acoustic driver and arrival of the acoustic energy at a
microphone associates with the noise reduction headphone;
FIG. 2B is as diagrammatic cross-sectional view of the
arrangement of FIG. 2A;
FIG. 3 is a plot of non-minimum phase delay;
FIG. 4 is a plot of magnitude response as a function of
frequency;
FIG. 5 is a plot of pattern of magnitude compensation as a
function of frequency; and
FIG. 6 is a plot of improvement of open loop gain of an
active noise reduction headphone employing the compensation pattern of FIG. 5
Though the elements of several views of the drawing may
be shown and described as discrete elements in a block diagram and may be referred to as "circuitry", unless otherwise
indicated, the elements may be implemented as one of, or a
combination of, analog circuitry, digital circuitry, or one or
more microprocessors executing software instructions. The
software instructions may include digital signal processing
(DSP) instructions. Some of the processing operations may
be expressed in terms of the calculation and application of
coefficients. The equivalent of calculating and applying coefficients can be performed by other analog or digital signal
processing techniques and those techniques are included
within the scope of this patent application.
Referring to FIG.lA, there is shown an active noise reduction headphone 110. The headphone includes two earphones
112, connected by a headband. Each earphone 112 may
include a cup shaped shel1114 and a cushion 116. The headband 117 exerts a force in an inward direction as represented
by arrows 119 so that the cushion 116 is urged against the
head of a user and surrounding the ear (typically referred to as
circumaural) to enclose a cavity which may include the outer
ear and ear canal; or urged against the ear ofthe user (typically
referred to as supra-aural) to enclose a cavity, which may
include the outer ear and ear canal; or urged into the ear canal
(typically referred to as interaural) to define a cavity, which

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US 8,054,992 B2

3

4

may include the ear canal. Interaural headphones may be
implemented without the headband, by inserting a portion of
the earphone into the ear canal. In the cavity are noise reduction elements that will be described below in the discussion of
FIG.1B.
Referring to FIG. 1B, there is shown a block diagram
illustrating the logical arrangement of a feedback loop in an
active noise reduction headphone. A signal combiner 30 is
combiningly coupled to a terminal 24 for an input audio
signal V1 and to a feedback preamplifier 35 and is coupled to
a compensator 37 which is in turn coupled to a power amplifier 32. Power amplifier 32 is coupled to acoustic driver 17 in
a cavity represented by dotted line 12. Acoustic driver 17 is
coupled to a combiner 36, as is terminal 25 which represents
noise P1 that enters cavity 12. The acoustic output P0 of
combiner 36 is applied to a microphone 11 coupled to output
preamplifier 35, which is in turn differentially coupled to
signal combiner 30.
Cavity 12 represents the cavity formed when an earphone
of a noise reducing headphone is pressed in, against, or
around a user's ear. Combiner 36 is not a physical element,
but represents the acoustic summation of noise P entering
1
cavity 12 from the external environment and acoustic ou~ut
radiated into cavity 12 by acoustic driver 17, the summation
resulting in acoustic energy P 0 being present in cavity 12.
Together, the acoustic elements of FIG. 1B, including the
microphone 11, the acoustic driver 17, and the cavity 12 may
be referred to as the "acoustic block" 100 which will be
discussed later.
In operation, an amplified error signal VE is combined
subtractively with input audio signal V1 at signal combiner 30.
The summed signals are presented to compensator 37. Compensator 37 provides phase and gain margin to me~t the
Nyquist stability criterion. Increasing the phase margm can
extend the bandwidth over which the system remains stable,
can increase the magnitude of feedback applied over a frequency range to increase active noise reduction, or both.
Aspects of compensator 37 will be discussed in more detail
below. Compensation, which includes applying a pattern in
which the magnitude varies with frequency, is similar to the
process called "equalization" and for the purposes of this
specification an equalization that is applied within feedback
circuit 10 is equivalent to compensation. There may be other
equalizations in the system; for example audi~ signal V1 may
be equalized prior to being applied to combmer 30. Power
amplifier 32 amplifies the compensated signal presented t.o
acoustic driver 17. Acoustic driver 17 transduces the amphfied audio signal to acoustic energy, which combines with
noise P entering cavity 12 to form combined acoustic energy
P 0 . Mi~rophone 11 transduces combined acoustic energy P 0
to an audio signal, which is amplified by preamp 35 and
presented subtractively as an error signal VE to signal combiner 30.
The closed loop transfer function of the circuit ofFIG.1 is

below) so that the phase angle ofEBDMA does not approach
-180° for any frequency at which IEBDMAisl. For
example, if the circuit is arranged so that at any frequency at
which IEBDMAisl, the phase angle is not more negative
than -135°, the phase margin is at least 180°-135° or 45°.
Stated differently, to maintain a typical desirable phase margin of no less than 45°, the phase angle of EBDMA at the
crossover frequency (the frequency at which the gain of
EBDMA is unity or 0 dB) should be ~-135°. Causing the
phase of transfer function EBDMA to be less negative in the
vicinity of the crossover frequency can allow an increase in
the crossover frequency, thereby extending the effective
bandwidth of the system.
Changes of phase angle as a function of frequency are a
result of at least two causes: time delays and phase shifts
associated with the magnitude of the transfer functions E, B,
D, M, and A, which may be frequency depe~dent. T~e
delays (for example delay ~t of FIG. 1 representing the t~e
delay between the radiation of acoustic energy by acoustic
driver 17 and the arrival of the acoustic energy at microphone
11) act as a phase shift that is linear as a function offrequency.
Other examples of time delays are delays in signal processing
components, particularly digital DSP systems such as the
components of FIG. 1. Phase shifts associated with transfer
functions E, B, D, M, and A are typically variable with respect
to frequency. It is desirable to reduce time delays and to
reduce or compensate for phase shifts associated with transfer
function EBDMA so that the phase angle of the circuit does
not approach -180° and preferably does not exceed -135° for
frequencies at which the magnitude of EBDMA exceeds
unity, or zero if expressed in dB.
Referring to FIGS. 2A and 2B, there are shown a top view
and a cross-sectional view taken along lines 2B-2B of FIG.
2A, respectively, of an arrangement that reduces the time
delay ~t (of FIG. 1) between the radiation of acoustic energy
by acoustic driver 17 and the arrival of the acoustic energy at
microphone 11'. An acoustic driver 17' includes a voice coil
43 mechanically coupled along a line 42 to a diaphragm 40.
The voice coil is typically tubular, and the attachment line 42
is typically circular, corresponding to one end of the tubular
form. The voice coil coacts with a magnetic structure 47 to
cause the voice coil to move linearly, in an intended direction
of motion, indicated by arrow 48. The voice coil 43 exerts a
force on diaphragm 40, causing diaphragm 40 to vibrate in the
direction indicated by arrow 48 to radiate acoustic energy.
Microphone 11 is positioned near diaphragm 40 along a line
49 intersecting attachment line 42 and parallel to the intended
direction of motion indicated by arrow 48. In some embodiments, microphone 11 is oriented with the opening 53 perpendicular to the direction of motion 48 and facing rad!ally
inward relative to the diaphragm 40. Preferably, the microphone 11 is placed so that the opening is within 2 llllll ofline
49 and may be aligned up with line 49. In the direction
indicated by arrow 48, microphone 11' is positioned as near as
possible to diaphragm 40 to minimize the time delay between
the radiation of acoustic energy from diaphragm 40, but not so
close as to interfere with the vibration of diaphragm 40 or to
negatively affect pressure gradient.
For purposes of illustration, microphone 11 is shown as
thin cylindrical microphones. Other types of microphones are
suitable.
An arrangement according to FIGS. 2A and 2B is advantageous because the time delay between the application of
force by the voice coil to the diaphragm along line 42 and the
radiation of acoustic energy (and therefore the time delay
between the application of force by the voice coil and the
arrival of acoustic energy at microphone 11') is less than the

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= l+EBDMA
60

where E, B, D, M, and A represent the frequency dependent
transfer functions of the compensator, the power amplifier,
the acoustic driver, the microphone, and the preamp, respectively. If the EBDMA term of the denominator=-! (the
equivalent of IEBDMAI=l and a phase angle of -180°) the
circuit becomes unstable. It is therefore desirable to arrange
the circuit so that the there is a phase margin (as described

65

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US 8,054,992 B2

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6

time delay if the microphone were placed at a position not
aligned with the attachment line 42 between the voice coil43
and the diaphragm 40, for example at point 52 over the center
of the diaphragm or point 50 over the edge of the diaphragm.
An arrangement according to FIGS. 2A and 2B may be
subject to frequency response aberrations such as peaks or
dips due to resonances of voice coil43. The aberrations may
be reduced by a number of methods. One method is to provide
a highly damped diaphragm, such as a diaphragm with laminar layers 58 and 60. In some implementations, top layer 58 is
polyurethane of average thickness 55 microns and lower layer
60 is polyetherimide of average thickness 20 microns.
Another method is to use stiffer material for the voice coil43
or provide stiffening structure 51 for the voice coil43 to shift
the resonant frequency out of the range of operation of the
acoustic driver.
FIG. 3 shows a plot (curve 62) of the non-minimum phase
delay (resulting from the time delay) as a function of frequency of a microphone placed at a point 50 (of FIG. 2A)
above the center of a diaphragm and a plot (curve 63) of a
microphone placed according to microphone 11' of FIG. 2A,
In the plot of FIG. 3, the phase delay is expressed as positive
degrees. The positive degrees of FIG. 3 are equivalent to
negative degrees in other sections of this specification. For
example, +40 degrees in FIG. 3 is equivalent to -40 degrees in
the discussion ofFIG.l.
FIG. 4 shows the magnitude response 68 as a function of
frequency of a typical acoustic block including acoustic
driver 17, microphone11, and cavity 12ofFIG.1. There is an
approximately 2nd order rolloffbetween 10 kHz and 20 kHz
and a very substantial 5'h or greater order roll off above 20
kHz. Or characterized differently, the curve has a low pass
shelving response shape between 10 kHz and 100kHz. Conventionally, the frequency range between 10 kHz and 100
kHz is considered of little importance, because for the most
part it is above the audible range of frequencies and because
it is more than a decade above the typical high crossover
frequency of active noise reduction headphone feedback
loops. However, the phase change associated with the steep
roll off above 10kHz may affect the phase angle of the feedback loop at frequencies in the audible range of frequencies.
FIG. 5 shows a pattern of magnitude compensation as a
function of frequency that may be applied by compensator 37.
Curve 70 represents a conventional compensation pattern,
with a slight roll off of compensation applied in the frequency
range between 10 kHz and 100 kHz. Curve 72 represents a
compensation pattern with a steeply increasing amount of
compensation applied in at least a portion of the frequency
range between 10 kHz and 50 kHz and up to 100 kHz. In the
range between 20kHz and 50 kHz and up to 100kHz, the
curve has a high positive slope (greater than 2nd order, for
example, 5'h order) on the same order as curve 68 rolls off.
The slope remains positive for at least an octave; for example
20kHz to 50 kHz is more than one octave and 20kHz to 100
kHz is more than two octaves. An example of a design for
such active noise reduction apparatus is given ina co-pending
patent application "Active Reduction Microphone Placing"
of Roman Sapiejewski, filed on the same day as this application and incorporated here by reference.
FIG. 6 shows the improvement in open loop gain of an
active noise reducing headphone (curve 78) employing the
compensation pattern of curve 72 of FIG. 5 over an active
noise reducing headphone (curve 76) using a conventional
compensation pattern, such as curve 70 of FIG. 5. The headphone employing the compensation pattern of curve 72 FIG.
5 provides more than an additional octave of bandwidth of
open loop gain.

The compensation pattern of FIG. 5 may be implemented
by an analog or digital circuit, but is most conveniently implemented as an analog filter including one or more operational
amplifiers with sufficient gain-bandwidth product and appropriately arranged resistors and capacitors and a power source.
Other implementations are within the scope of the claims.
What is claimed is:
1. A feedback circuit for an active noise reduction headphone comprising: an acoustic block characterized by a first
magnitude frequency response; a compensator characterized
by a second magnitude frequency response to combine the
second magnitude frequency response with the first magnitude frequency response to provide a combined magnitude
frequency response, wherein the second magnitude frequency response is characterized by a first pattern that has a
positive slope at a frequency interval in the spectral portion
above 10kHz so that the phase shift of the combined magnitude frequency response of the feedback circuit at frequencies
in the audible range of frequencies is less than the phase shift
of the combined magnitude frequency response of the feedback circuit in the audible range of frequencies wherein the
second magnitude frequency response characterized by a second pattern that does not have a positive slope in the spectral
portion above 10 kHz.
2. A feedback circuit in accordance with claim 1, wherein
the pattern has a positive slope between 20 kHz and 50 kHz.
3. A feedback circuit in accordance with claim 2, wherein
the pattern has a positive slope between 20kHz and 100kHz.
4. A feedback circuit in accordance with claim 1, wherein
the compensator comprises a digital filter.
5. A feedback circuit in accordance with claim 1, wherein
the compensator comprises an analog filter.
6. A feedback circuit in accordance with claim 1, wherein
the positive slope is second order or more.
7. A feedback circuit in accordance with claim 6, wherein
the positive slope is fifth order.
8. A method comprising: in an active noise reduction headphone comprising a feedback loop characterized by a magnitude frequency response, compensating the magnitude frequency response by a first pattern that has a positive slope
between 20 KHz and 50 kHz to provide a compensated magnitude frequency response so that the phase shift of the compensated magnitude frequency response at frequencies in the
audible range of frequencies is less than the phase shift of the
compensated magnitude frequency response in the audible
range of frequencies wherein the compensating comprises
compensating the magnitude frequency response by a second
pattern that does not have a positive slope in the spectral
portion above 10 kHz.
9. A method in accordance with claim 8, wherein the compensating comprises compensating the magnitude frequency
response by a pattern that has a positive slope between 20kHz
and 100kHz.
10. A method in accordance with claim 8, wherein the
positive slope is second order or more.
11. A method in accordance with claim 10, wherein the
positive slope is fifth order.
12. A compensation pattern for an active noise reduction
headphone comprising a feedback loop characterized by a
magnitude frequency response, compensating the magnitude
frequency response by a first pattern that has a positive slope
in the frequency range between 20KHz and 50 kHz to provide
a compensated magnitude frequency response so that the
phase shift of the compensated magnitude frequency
response at frequencies in the audible range of frequencies is
less than the phase shift of the compensated magnitude frequency response in the audible range of frequencies wherein

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the compensating comprises compensating the magnitude
frequency response by a second pattern that does not have ~
positive slope in the spectral portion above 10 kHz.
13. A compensation pattern in accordance with claim 12
further characterized by a positive slope in the frequenc;
range between 20KHz and 100kHz.
14. A compensation pattern in accordance with claim 12
further characterized by a greater than 2nd order positive slop~
between 20kHz and 100 kHz.
15. A compensation pattern for an active noise reduction
headp.hone comprising a feedback loop characterized by a
magmtude frequency response compensating the magnitude
frequency response by a first pattern that has a positive slope
above 10kHz for a range of at least one octave to provide a
compensa~ed magnitude frequency response, so that the
phase shift of the compensated magnitude frequency
response at frequencies in the audible range of frequencies is
less than the phase shift of the compensated magnitude frequency response in the audible range of frequencies wherein
the compensating comprises compensating the magnitude
frequency response by a second pattern that does not have a
positive slope in at least a portion of the spectral range above
10 kHz for at least one octave.
16. A compensation pattern in accordance with claim 15
characterized by a positive slope for a range of at least tw~
octaves.
17. A compensation pattern in accordance with claim 16
characterized by a positive slope for a range of at least thre~
octaves.
18. A method comprising: providing an active noise reduction h~dphone comprising a feedback loop characterized by
a .magmtude frequency response; and compensating the magrutude frequency response by a first pattern that has a positive
slope in at least a portion of the spectral range above 10 kHz
for at least one octave to provide a compensated magnitude
frequency r~sponse, so that the phase shift of the compensate_d magrutude frequency response at frequencies in the
audible range of frequencies is less than the phase shift of the

compensated magnitude frequency response in the audible
range of frequencies wherein the compensating comprises
compensating the magnitude frequency response by a second
pattern that does not have a positive slope in at least a portion
of the spectral range above 10kHz for at least one octave.
19. A method in accordance with claim 18 wherein the
compensating comprises compensating the ~gnitude frequency response by a pattern that has a positive slope above
10kHz for at least two octaves.
20. A method in accordance with claim 18, wherein the
compensation comprises compensating the magnitude frequency response by a pattern that has a positive slope above
10kHz for at least three octaves.
21. A method for increasing phase margin in a feedback
circ~it.of an active J?-Oise reduction headphone comprising:
prov1dmg an acoust1c block comprising an acoustic driver
comprising a voice coil mechanically coupled along an
attachmen~ line to an acoustic energy radiating diaphragm,
t?e acoust1c bl?ck further comprising a microphone posit10ned along a !me parallel to an intended direction of vibration of the acoustic diaphragm and intersecting the attachment line, the acoustic block characterized by a magnitude
frequency response; compensating the magnitude frequency
response by a first compensation pattern that has a positive
slope over at least one spectral range above 10kHz so that the
phase shift of the combined magnitude frequency response of
the feed~ac~ circuit at frequencies in the audible range of
:U:equenc1es 1s less than the phase shift of the combined magrutude frequency response of the feedback circuit in the
audible range of frequencies wherein the second magnitude
frequency response characterized by a second pattern that
does not have a positive slope in the spectral portion above 10
kHz.
22. A method in accordance with claim 21 wherein· the
positive slope is second order or more.
'
23. A method in accordance with claim 22, wherein the
positive slope is fifth order.

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